#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <string>
#include <queue>
#include <stack>
#include <math.h>
#include <set>
#define ALL(obj) (obj).begin(),(obj).end()
#define RALL(obj) (obj).rbegin(),(obj).rend()
#define P pair<int, int>

#define MOD 1000000007
#define INF 1012345678
#define NINF (-2147483647-1)
#define LLINF 9223372036854775807
using ll = long long;
using namespace std;

int main() {
    int N, D;
    cin >> N >> D;
    vector<int> Tmax(N), Kmax(N);
    cin >> Tmax[0] >> Kmax[0];
    Kmax[0] -= D;
    for (int i = 1; i < N; i++)
    {
        int T, K;
        cin >> T >> K;
        Tmax[i] += max(Tmax[i - 1], Kmax[i - 1] - D) + T;
        Kmax[i] += max(Kmax[i - 1], Tmax[i - 1] - D) + K;
    }
    cout << max(Tmax[N - 1], Kmax[N - 1]) << endl;
    getchar(); getchar();
    return 0;
}